自动化冗余通孔插入：强化先进制程互连可靠性

As the semiconductor industry pushes deeper into the sub-5nm realm, the physical design landscape has shifted from being dominated by gate delays to being defined by interconnect performance. In modern system-on-chip (SoC) designs, the wiring that connects billions of transistors has become the primary bottleneck for power, performance, and area (PPA). Among the various components of the interconnect stack, vias – the vertical connections between metal layers – represent one of the most significant points of vulnerability.

At advanced process nodes, via resistance increases exponentially as dimensions shrink. This rising resistance, coupled with higher current densities, exacerbates challenges related to electromigration (EM) and IR drop. Traditionally, design teams addressed these issues during the final signoff phase, often resulting in late-stage engineering change orders (ECOs) that could delay a project by weeks. To maintain aggressive time-to-market schedules while ensuring long-term silicon reliability, a new approach is required: shifting signoff-aware interconnect optimization into the implementation phase.

The physics of via failure and interconnect resistance

To understand the need for automated via optimization, first consider the physical challenges at play. In a typical advanced-node interconnect, the resistance of a single via can be significantly higher than that of a corresponding length of metal wire. As current flows through these narrow vertical structures, the resulting heat and atomic displacement can lead to electromigration – the gradual movement of metal atoms that eventually creates voids or shorts.

Plus, the cumulative effect of via resistance across a massive power delivery network (PDN) leads to IR drop. If the voltage reaching a logic gate falls below a certain threshold, the gate’s switching speed slows down, potentially leading to timing failures. In the worst cases, the gate may fail to switch entirely.

Historically, designers mitigated these risks by manually adding redundant vias to critical signal paths and power grids. The redundant vias provide parallel paths for current, lowering the effective resistance and reducing the current density through any single via. However, at 5nm and below, the sheer density of the layout and the complexity of foundry design rules make manual insertion practically impossible. Figure 1 illustrates the problem.

Fig. 1: The increasing complexity of advanced-node layouts requires automated solutions to identify valid locations for redundant via insertion without violating foundry rules.

The limitations of traditional implementation flows

Most place-and-route (P&R) tools include basic capabilities for via redundancy. However, these routers are primarily optimized for connectivity and timing closure, not for the exhaustive reliability enhancement required for high-performance silicon. Router-based via insertion typically comes with several limitations:

1. Lack of signoff awareness: Implementation routers typically use simplified versions of foundry design rules to maintain performance. While this is sufficient for initial routing, it often leads to DRC violations when the design is subjected to the full signoff rule deck.
2. Limited search space: Routers often give up on via insertion in congested areas to avoid creating shorts. This leaves critical parts of the design – often the very areas that need the most help – under-optimized.
3. ECO overhead: When a router-based enhancement triggers a signoff violation, the resulting ECO cycle requires re-running parts of the implementation flow, which consumes valuable time and can disrupt timing closure.

To overcome these hurdles, many design teams adopt a “shift-left” methodology that uses signoff-quality engines earlier in the design cycle. By integrating the same logic used for final verification into the enhancement process, designers can achieve “correct-by-construction” results that are guaranteed to pass final signoff.

Methodology: Automated signoff-aware enhancement

The shift-left approach to via optimization involves a specialized enhancement platform that operates on the design database during the implementation phase. This platform reads standard design formats – such as LEF/DEF or OASIS – and applies a multi-step process to strengthen the interconnect.

The first step is a comprehensive analysis of the existing layout to identify every potential location where a redundant via could be placed. This is not a simple proximity check; the tool must account for complex multi-patterning constraints, voltage-aware spacing rules, and enclosure requirements. By leveraging a signoff-grade geometry engine, the tool can navigate these constraints with a level of precision that far exceeds traditional routers.

Once the valid insertion points are identified, the tool strategically adds redundant vias to the user-selected nets. This can be applied globally to the entire power grid or targeted toward specific high-priority signal nets, such as clock trees or long-distance bus lines. The result is a layout that is significantly more robust, with lower overall resistance and improved EM margins.

Fig. 2: By moving signoff-aware optimization into the implementation phase, design teams can resolve interconnect issues before they become late-stage bottlenecks.

Navigating layout imperfections with short-resilience

One of the most innovative aspects of modern automated enhancement is the ability to handle “dirty” or incomplete layout data. A typical design database is rarely perfectly clean during the implementation flow. Localized shorts, routing overlaps, or incomplete power grid definitions are common as the design matures.

Traditional optimization tools often fail when encountering these imperfections, either crashing or producing illegal layout modifications. A robust enhancement platform must be “short-resilient.” This means the tool can identify regions with existing connectivity conflicts and automatically designate them as keep-out areas (Figure 3).

Fig. 3: Short-resilient DE-Via flow; avoiding insertion on identified short-prone regions.

By protecting these suspicious regions, the tool can continue to optimize the rest of the design. This lets design teams run via enhancement early and often, even before the layout is fully DRC-clean. This iterative approach is essential for maintaining productivity in a fast-paced design environment, as it prevents minor layout issues from stalling the overall reliability enhancement of the chip.

Impact on power delivery and electromigration

The technical benefits of automated via insertion are most evident in the power delivery network. In high-performance SoCs, the PDN must deliver massive amounts of current with minimal voltage drop. As the number of metal layers increases, the vertical resistance through the via stacks becomes a dominant contributor to the total IR drop.

By maximizing the number of vias in the PDN, automated enhancement significantly lowers the vertical impedance of the grid. This leads to a more uniform voltage distribution across the die, providing more timing margin for the logic gates. In many cases, this enhancement can be the difference between meeting a performance target and having to down-clock the final product.

From an EM perspective, additional vias provide a critical safety net. Electromigration is a statistical phenomenon; while a single via might be designed to handle a specific current, manufacturing variations can lead to localized “hot spots” where the current density exceeds the safe limit. By providing parallel paths, automated insertion ensures that the current is distributed more evenly, reducing the likelihood of a catastrophic failure over the lifetime of the device.

Scalability and integration in modern SoC flows

As SoC designs grow to include billions of transistors and tens of kilometers of wiring, the scalability of the enhancement platform becomes vital. The tool must be able to process massive data volumes quickly enough to fit within a standard overnight implementation run. This requires a highly parallelized architecture that can distribute the layout analysis and insertion tasks across hundreds of CPU cores.

Furthermore, the platform must integrate seamlessly into the existing EDA ecosystem. By supporting standard industry formats and providing clear reporting on the enhancements made, the tool allows designers to maintain full control over their layouts. The ability to run “what-if” scenarios – such as testing the impact of different via densities on IR drop – provides designers with the insights needed to make informed trade-offs between PPA and reliability.

Conclusion: The future of design robustness

In the advanced-node era, the distinction between implementation and verification is blurring. The challenges of 5nm, 3nm, and beyond require that signoff awareness be present at every stage of the design cycle. Automated via insertion is a prime example of how this shift-left methodology can provide tangible benefits in terms of silicon reliability and schedule predictability.

By automating a traditionally manual and error-prone process, design teams can focus their efforts on higher-level architectural innovations, secure in the knowledge that their interconnects are built on a foundation of signoff-verified robustness. As interconnect challenges continue to mount, the adoption of automated, signoff-aware enhancement platforms will become not just an advantage, but a necessity for any team aiming to deliver high-performance silicon in the modern era.